We have been developing various experimental models in hamsters. Our primary interest is female protein (FP), a serum pentraxin which is under sex steroid control in the Syrian hamster. Although FP is very similar structurally to human homologs (serum amyloid P-(SAP) and C-reactive protein (CRP)), this protein as expressed in Syrian hamsters has a number of unique features. However, even in related hamsters (Turkish, Armenian, Dunzgarian) FP expression is under different control than in Syrian, although the protein appears to be very similar structurally. In fact, so similar that hybrid pentameric FP molecules have been constructed in vitro. Thus, in Armenian hamster, FP is not sex limited as male-female serum levels are similar; in contrast to Syrian hamsters, Armenian FP hepatic synthesis is not suppressed by testosterone, although exogenous estrogens including DES, will down regulate synthesis. Estrogens have another effect on Armenian hamster liver, and that is an acute toxicity which is clinically manifest as a profound jaundice which is detectable within two days of estrogen administration. Of particular interest is the finding that concomitant administration of tamoxifen, an estrogen receptor blocking agent, will inhibit this hepatotoxic effect of estrogens, indicating an estrogen receptor is mediating this liver injury. The role of FP in this hepatotoxicity is unknown, although, low FP levels are only found in those two species which are susceptible the Armenian and Chinese hamsters.